Spectroscopic analysis generally relies on detection and quantification of emission, absorption, or scattering of radiation by matter. In the case of gas-phase spectroscopy, the emission, absorption, or scattering of radiation occurs by individual molecules of one or more compounds present along a radiation path between a radiation source and a detector. At least some of the radiation transmitted along this path can be absorbed or scattered, or other radiation can be emitted by the molecules in the radiation path. The wavelength of the absorbed, scattered, or emitted light generally is determined by the particular energy transition occurring to the molecules of one or more emitting or absorbing compounds in the radiation path. For example, in infrared spectroscopy, discrete energy quanta are absorbed by molecules due to excitation of vibrational or rotational transitions of the intra-molecular bonds.
Some uses of spectroscopic analysis techniques involve characterizing the presence and/or concentration of one or more target analyte compounds in a gas sample. Analyzers based on spectroscopic analysis are used in a variety of applications, including but not limited to process monitoring, process control, energy content metering, detection of chemical contaminants in gas streams, and the like. These applications generally involve repeated, sequential analysis of a flowing gas stream to detect and/or quantify one or more compounds in the gas stream. A typical configuration of such an analyzer receives a flowing supply of the gas stream into a sample volume through which a radiation path between one or more sources and one or more detectors is directed one or more times. In some examples, one or more mirrors are included in the radiation path such that the radiation path traverses at least part of the sample volume more than once.
A sample volume can in some examples be at least partially enclosed in a sample cell through which the radiation path passes at least once. It can be desirable to maintain the flow rate of the supplied gas stream through the sample cell (or other sample volume) at a sufficient flow rate to prevent sample contamination by mixing of one or more other gases (e.g., ambient air, etc.) with the sample gas, as well as to provide pressure and temperature stability to the sample gas for an accurate measurement. Flow rate control can also be desirable for preventing uncontrolled or excessive releases of potentially toxic or unhealthy or environmentally undesirable gases into the atmosphere. Flow rate control can furthermore be important to achieve improved response times to changes in the gas stream.
In various spectroscopic analyzer implementations, the structure and shape of a sample cell can be constrained by mechanical considerations. For example, a sample cell can desirably meet one or more structural requirements necessary to maintain a relative position of a radiation source and a detector to improve calibration accuracy and fidelity in gas measurement applications. Movement of the detector relative to the radiation source or other changes in the radiation path between the radiation source and the detector can lead to deviations in analyzer performance, analytical errors, lack of reproducibility, etc. Sample cell design can also include considerations relating to temperature stability and control, gas flow control, and the like.